Many industries employ remote sensing to provide information collected from multiple remote locations to a single display. One example of a remote sensing system is a moisture sensing system. The moisture sensing system includes a plurality of moisture sensors that are placed at different locations in a garden, field, or any location where moisture detection is desired. Each moisture sensor is configured to perform one or more moisture measurements including the moisture level of soil and humidity measurements. Each of the moisture sensors further includes a transmitting device, typically a radio transmitter, that transmits signals to a receiving station. The signals transmitted from each sensor include data that represent the moisture measurements from each of the corresponding sensors. The receiving station typically includes a display that can inform a user of the moisture levels detected by the remote moisture sensors. Some receiving station embodiments include visual or audible alarms that can inform an operator if detected moisture levels either exceed or fall below a desired range in the location of one or more sensors. Various receiving station embodiments additionally include electronic data collection devices to store a history of detected moisture levels generated by the moisture sensors over time.
Remote sensing networks, including moisture sensing networks, face challenges in operation. One such challenge is that remote sensor networks may be deployed in a dense configuration that can lead to false sensor signals. For example, if sensor network A is deployed in one field, while sensor network B is deployed in a second field that is across a road from the first field, then the transmitters coupled to the moisture sensors in network A may generate signals that are confused with the transmitters of network B, and vice-versa. The proximity of the sensor networks can lead to interference or false moisture readings at the corresponding receivers for each of the sensor networks. Even if only a single sensor network is deployed in one location, the transmitters from different sensors in the network may interfere with each other as well.
Prior art radio networks include techniques including carrier sensing multiple access (CSMA) with collision avoidance and collision detection techniques to mitigate the problems described above. Additionally, cellular data networks are known that use numerous modulation techniques to enable multiple cellular telephones to operate simultaneously. Many sensor networks, however, are not well suited to using these techniques. In a typical sensor network, the sensors and associated transmitters are designed to be low cost and to operate for long periods of time using a battery. Since the sensors are exposed to the environment, any electronics need to be rugged and capable of operating under a wide range of weather conditions for a prolonged time period. Consequently, the transmitters often include low cost electronics and one-way radio transmitters that are not capable of performing known multiple access techniques without requiring costly design changes or reductions in battery life and reliability. Therefore, techniques to improve the operation of sensor networks with multiple transmitters that transmit data to a single receiver would be beneficial.